Conventional cathode ray tubes suitable to color television include an envelope having a viewing area with an inner surface covered by phosphors to provide a correct color and image rendition when impinged by an electron beam emanating from an electron gun. Disposed intermediate the electron gun and the inner surface of the viewing area is an aperture mask having a plurality of closely spaced apertures. These apertures are of a dimension and configuration such that ideally the electron beam is prohibited from striking phosphors other than those which provide the correct and desired color and image rendition.
Normally, the above-mentioned aperture masks are made from a metallic material, such as steel, and the apertures are effected by way of well-known photochemical machining process. Also, it has been a common practice to utilize a so-called phosphor-dot type screen wherein the aperture mask included a plurality of substantially circular holes whereby the phosphor dots were deposited onto the inner surface of the viewing area.
Accordingly, the dimensions of the holes and the resultant phosphor dots were critical in order to provide proper impingement of the phosphor dot by an electron beam. To insure the correctness of these critical dimensions, one technique provided a magnification apparatus whereby an inspector visually checked each aperture mask and passed or rejected the part. Obviously, such a technique is not only cumbersome and slow but also subject to operator error and judgement decisions.
Following, it was found that light transmission of the holes in the aperture mask provided information sufficient to accurately determine the hole size. Thus, the undesired relatively slow and relatively inaccurate magnification apparatus employing an operator and relying upon operator judgement was rendered obsolete and a faster more accurate light transmission technique evolved.
However, the phosphor-dot type of cathode ray tube is being rapidly replaced by the so-called "slotted" mask type of cathode ray tube structure. Therein, the apertures are in the form of slots, rather than holes, and the phosphors are in the form of stripes rather than dots. Thus, the new slotted arrangement requires measurements of both length and width if correct dimensional configurations are to be determined as compared with the relatively simple prior known holes wherein a single diametrical measurement was sufficient.
In an attempt to provide satisfactory measurements of the so-called "slots", a return was made to the so-called magnification process wherein a microscope and an operator were used to determine one dimension of the slot. Then, light transmission of the total slot was combined with this one microscopic measurement to determine the remaining dimension or width of the slots of the slotted aperture mask.
Although the above-mentioned technique has been and probably still is used with varying degrees of success, it has been found that the results leave something to be desired in so far as both accuracy and measurement efficiency are concerned. More specifically, it has been found that the technique again relies upon relatively inconsistent and inaccurate operator judgement. Moreover, undesired but relatively common variations and rounded ends of the slots render operator judgement and resultant accuracy difficult if not impossible.